Abbreviations

Parallel Port Signals

Some signals are electrically inverted, i.e., a logical 'true' is
indicated by low level on the physical line/pin.

It may be confusing that some line states are sometimes inverted on
the PC's port registers. This is totally independent from the electrical
inversion. This whole document will only talk about physical voltage
levels, so the port inversion is only important when programming kernel
drivers and the like.

The electrical inversion is indicated in the signal name by an
initial slash, e.g. /Init.
The (maybe addititional) PC port inversion is shown in the
following table.

The pin number given in the table refers to the sub-D 25 pin connector.

Example

When a PC wants to indicate a reset condition, it has to set the
/Init line to low level, because
it is electrically inverted (as shown in the name). Because the PC
has positive logic on that pin, it has to write a 0 into the
corresponding bit in the register.

Note

The host-to-peripheral signal /Select is sometimes
called /Select_in to clearly distiguish it from
the peripheral-to-host signal Select.

Line States

In contrast to normal convention, signal timing diagrams will
be shown in top-down direction instead of left-to-right. This has
the advantage that explanatory text can easily be provided for
each step.